1. Field of the Invention
The present invention relates to recording (printing) methods and apparatuses for driving a recording head (print head) having a nozzle array.
2. Description of the Related Art
The recent development of personal computers has been accompanied by dramatic advances in printer technology which have enabled high-quality image outputs. Such high-quality recording (printing) may require performing a registration process (e.g., calibration process) to correct misalignment problems.
Various techniques have been implemented to correct a misalignment of positions where ink droplets of different colors are landed or positions where ink-droplets of the same color are landed in opposite scanning directions of a recording head in bidirectional printing.
One common type of recording heads (print heads) is a color print head having two nozzle arrays for each color, a large-dot nozzle array and a small-dot nozzle array, for example, as shown in FIG. 2. The number of nozzles may be increased to achieve higher print speeds. In the example of FIG. 2, the large-dot and small-dot nozzle arrays each include 192 nozzles.
Japanese Patent Laid-Open No. 2000-71433 discloses one of the generally used methods for driving a print head. In this method, nozzle arrays extending in a column direction (sub-scanning direction) are each divided into nozzle blocks every predetermined number of nozzles. These blocks are driven at different timings. The time-division driving allows for increased ink-supply speed and stability and reduced power consumption required for ejection.
FIG. 3 is a table showing a configuration of an example of a nozzle array divided into 16 blocks. As shown in this table, nozzles are grouped into blocks every 16 nozzles. That is, nozzles separated at intervals of a predetermined number of nozzles are grouped into the same block so that the adjacent nozzles can be driven with a reduced effect on each other.
Japanese Patent Laid-Open No. 2001-129985 discloses a registration method for correcting a misalignment of positions where ink droplets of different colors are landed or positions where ink droplets of the same color are landed in opposite scanning directions in bidirectional printing.
Japanese Patent Laid-Open No. 5-84899 discloses a method for correcting a misalignment of positions where ink droplets are landed at different print speeds. In this method, the time intervals (t) for time-division driving are controlled according to the print speeds. That is, the time intervals (t) are decreased for high-speed printing and are increased for low-speed printing to avoid a misalignment of positions where ink droplets are landed.
Some printers include a memory (RAM) with a limited memory capacity for cost reduction. Such printers generally lower the resolution of print data in a main scanning direction because of the limited memory capacity, and thus repeat scanning in the main scanning direction (multipass printing) for dot interpolation. According to this method, if the actual print resolution of data stored in a memory of a printer is 600 dpi, scanning may be repeated eight times at 600 dpi in multipass printing to achieve a print resolution of 4,800 dpi in the main scanning direction.
If recording image data with a relatively low actual resolution, for example, 600 dpi, in the main scanning direction is printed directly at the recording resolution, a column of image data is distributed at 600 dpi by time-division driving. In conventional printing methods, therefore, the positions where dots are recorded can be adjusted only in units of 600 dpi, and nozzle registration can be performed only in units of 600 dpi accordingly.
In addition, recording methods adapted for recording heads having increased numbers of nozzles have been in demand.
Furthermore, the inclination (misalignment) of a nozzle array can be caused by variations in the production of print heads or a poor fit between a print head and a carriage provided on a printer. Thus, a deviation in ejection direction between nozzles (for example, nozzles 0 and 191 in FIG. 2) may need to be corrected.
Registration methods are typified by the shift of print data by a plurality of pixels, or a half of a pixel, of a recording resolution (print resolution) in units of a column and the shift of the reference timing for printing by a predetermined period.
The shift of print data by a plurality of pixels in units of a column, for example, is intended to roughly correct a misalignment of positions where ink droplets of different colors are landed or positions where ink droplets of the same color are landed in opposite scanning directions in bidirectional printing. The data shift can be performed in units of 600 dpi for printing at 600 dpi. The data shift can also be performed by a half of a pixel of a print resolution, 1,200 dpi for the above example. The shift of the reference timing for printing by a predetermined period is performed within the period corresponding to one column. The print timing can be shifted in units of a basic clock cycle for operation of a printer system. This type of shift is intended to correct a slight misalignment due to, for example, variations in individual head products and differences in recording environments.
Although the position where printing is started can be shifted in the methods described above, the time intervals for time-division driving in ejection from nozzle blocks of each column are not changed. If, for example, printing is performed at a print speed (carriage speed) of 40 inch/sec and a resolution of 600 dpi, the time required for ejection from all nozzles of each column is represented by the following equation:Tcolumn=(1/40(inch/sec))/600(dpi)=41 μs One of the commonly employed methods for defining the time intervals for ejection for each column is the use of an encoder disposed on a carriage to read a scaler extending in the direction in which the carriage moves. Using this method, ejection intervals are uniformly defined for the individual columns in a print region where the carriage moves at a constant speed.
The time intervals for driving the blocks of each column are determined by dividing the time interval for ejection for each column by the number of blocks. Under the above conditions, for example, the time interval for ejection from each of the 16 blocks is represented by the following equation:Tblock=Tcolumn/16(blocks)=2.60 μs 
Thus, time intervals are uniformly defined both for columns and for blocks according to the reference timing for printing. The time interval for ejection for each column is not changed because only the timing when ejection is started is shifted by registration. For conventional printers, the time interval for ejection for each column in a raster direction depends on the print speed of a carriage and print resolution. In practice, however, the time interval for ejection for each column can be assumed to depend on print resolution because the print speed of a carriage is limited to several modes based on the optimum ejection frequency of a print head.
Recent technology has enabled the ejection of significantly reduced volumes of ink droplets, namely, 1 to 2 pl, to achieve printing comparable in quality to film photography. Such fine ink droplets form fine dots on paper. Conventional time-division driving can cause time differences between blocks and thus may fail to align the ejection positions of all nozzles. For conventional print heads, a misalignment of dots formed by time-division driving within a column is negligible because the print heads eject larger volumes of ink droplets, namely, 20 to 50 pl, to form dots overlapping each other on paper.
For the example described above, the time difference between the first block and the sixteenth block is calculated to be about 39 μs. A misalignment of ink droplets with a volume of 2 pl cannot be recognized by the human eye, but can be recognized as a fringe pattern in an image formed on paper.